An IsString (NonEmpty Char) instance

[Ivan Lazar Miljenovic]

On 3 September 2017 at 19:33, Tony Morris <tonymorris at gmail.com> wrote:
> Why not:
>
> class isString1 a where fromString1 :: NonEmpty Char -> a

It's backwards-incompatible, but my preference - especially when you
consider the libraries that parse the String - is to have fromString
return Maybe.

>
>
> On 02/09/17 22:08, Artyom wrote:
>
> tl;dr
>
> We have instance IsString [Char] and instance IsList (NonEmpty a). Let’s
> also have IsString (NonEmpty Char).
>
> Background
>
> IsString is a class that is used with the -XOverloadedStrings extension to
> support string literals of types other than String - for instance, with the
> IsString Text instance in scope, you can write "foobar" :: Text and it will
> compile.
>
> For reference, the standard libraries supply the following instances of
> IsString:
>
> instance (a ~ Char) => IsString [a]
> instance IsString a => IsString (Const a b)
> instance IsString a => IsString (Identity a)
>
> Proposal
>
> I propose adding a new instance of IsString for NonEmpty lists of
> characters. NonEmpty has been in base starting from version 4.9, and for
> that reason people are starting to use it more often - e.g. the popular
> megaparsec library defines its custom error type like this:
>
> data ErrorItem t
>   = Tokens (NonEmpty t)      -- ^ Non-empty stream of tokens
>   | Label (NonEmpty Char)    -- ^ Label (cannot be empty)
>   | EndOfInput               -- ^ End of input
>
> Here NonEmpty Char stands for “non-empty string”. Without the IsString
> instance users are forced to write non-empty strings in an inconvenient and
> awkward-looking way (e.g. Label ('f' :| "oobar")); the instance makes Label
> "foobar" an acceptable notation, thus making the NonEmpty Char type more
> viable for use in libraries and user-facing APIs.
>
> Implementation
>
> Here’s a sample implementation:
>
> instance (a ~ Char) => IsString (NonEmpty a) where
>   fromString (a:as) = a :| as
>   fromString "" = errorWithoutStackTrace "NonEmpty.fromString: empty string"
>
> This mirrors the IsList instance for NonEmpty. (The reason I haven’t used
> fromList is that I want the error message to say “fromString” instead of
> “fromList”.)
>
> If this is accepted, I can make a patch.
>
>
>
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-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
http://IvanMiljenovic.wordpress.com